1. Field of the Invention
The present invention relates to an image forming apparatus including an image forming portion for forming an image on a sheet fed from a sheet loading portion and a re-feeding path for re-feeding the sheet, on which an image has been formed by the image forming portion, to the image forming portion.
2. Related Background Art
Conventionally, as a method of forming images on both sides of a multiplicity of recording sheets, there is a method of utilizing a circulating type sheet transporting process (what is called a tray-less both side mechanism or a through-pass both side mechanism). With this circulating type sheet transporting process, a recording sheet is sent to a transfer portion for transferring an image, then, the recording sheet having an image transferred on one side is sent to a reversing portion, the recording sheet reversed in the reversing portion is sent again (re-fed) to the transfer portion without being gathered in a tray for exclusively containing sheets to be printed on both sides, and the image is transferred on the opposite side.
The both side image forming method employing this process has advantages compared with a both side image forming method, with which a recording sheet having an image formed on one side is revered, all of reversed sheets are gathered in a tray for exclusively containing sheets to be printed on both sides, the gathered sheet is sent again to a transfer portion to thereby form an image on the other side (what is called a stack type both side mechanism). That is, the speed for forming an image is improved, unnecessary increase of costs due to installation of the tray for exclusively containing sheets to be printed on both sides or the like and a limitation on the number of loaded sheets in the tray for exclusively containing sheets to be printed on both sides are not caused.
As an image forming apparatus employing such a circulating type sheet transporting process, there is an apparatus disclosed, for example, in Japanese Patent Application Laid-open No. 9-315699. The disclosed image forming apparatus performs such control that, if the number of sheets on the both sides of which an image should be formed exceeds the number of sheets circulating in the apparatus, sheets are fed firstly in the number of xe2x80x9c(number of circulating sheets+1)/2xe2x80x9d (i.e., in the case of three sheets circulating type image formation, (3+1)/2=2, in the case of five sheets circulating type image formation, (5+1)/2=3, and in the case of seven sheets circulating type image formation, (7+1)/2=4), and then, sheet feeding and sheet re-feeding are alternately repeated, whereby images can be formed at the same interval for feeding sheets as in one side image formation.
In addition, as a method of preventing a jam by executing transportation control according to a type of a sheet, there is a method disclosed, for example, in Japanese Patent Application Laid-open No. 9-278252. With the disclosed method, a paper type sensor is provided in an image forming apparatus, which determines a paper type and causes the image forming apparatus not to apply straight paper discharge or both side printing to sheets such as cardboard and an OHP sheet.
FIGS. 20A to 20C are schematic illustrations showing timings for feeding sheets according to different numbers of circulating sheets in an identical path length in an image forming apparatus of this type.
Further, a frame of solid lines not hatched indicates feeding of a front side of a recording sheet in both side image formation or feeding of a recording sheet in one side image formation. In addition, a hatched frame of solid lines indicates re-feeding of a back side of a recording sheet in the both side image formation. A frame of broken lines indicates that there is a space equivalent to one sheet between sheets.
In addition, in the figure, numerals 1 to 5 written on recording sheets indicate how many sheets have been fed.
In the figures, FIG. 20A corresponds to circulation of three sheets of a large sheet size (e.g., A3 size), FIG. 20B corresponds to circulation of three sheets of a small sheet size (e.g., A4 size), and FIG. 20C corresponds to circulation of five sheets of a small sheet size (e.g., A4 size).
As shown in the figure, when a length of sheet path 19001 is fixed, if three sheets circulating type image formation is performed in the case of sheets of a large size as shown in FIG. 20A, and if five sheets circulating type image formation is performed in the case of sheets of a small size as shown in FIG. 20C, images can be formed efficiently with only a few spaces between sheets. However, in the case of sheets of a small size with the identical sheet path length 19001 as shown in FIG. 20B, if the three sheets circulating type image formation is performed, there are many spaces between sheets and images cannot be formed efficiently.
In the above-mentioned conventional process, the number of circulating sheets peculiar to an image forming apparatus is fixed. Thus, there is a problem in that, if an image forming apparatus capable of circulating three sheets of a large size to form images as shown in FIG. 20A performs a circulating operation with sheets of a small size as shown in FIG. 20B, there are many space between sheets, and a throughput decreases.
In addition, in recent years, the both side printing can also be performed in an image forming apparatus, which prohibited reverse discharge and both side printing of cardboard before. However, there is also a problem in that, if the number of circulating sheets is fixed depending on an apparatus or if the number of circulating sheets is made determinable according to a sheet size in order to overcome the above-mentioned problem, there is a possibility that a transporting speed of sheets is slowed down and a jam occurs due to shortage of torque of a transporting motor depending on a type of a sheet such as cardboard when the number of circulating sheets is identical with that of plain paper.
The present invention has been devised in order to solve the above-mentioned problems. It is an object of the present invention to provide an image forming apparatus for forming images on both sides of a sheet by a circulating type sheet transporting process, a method of controlling the image forming apparatus and a storage medium for the image forming apparatus wherein a user interface capable of setting a type of a sheet is provided to perform both side image formation in which the number of circulating papers is variable according to not only a size of a sheet but also a type of a sheet at the time of the both side image formation by the circulating type sheet transporting method, whereby transportation control is performed according to a type of a sheet on which images are formed and occurrence of sheet curl and a jam due to the sheet curl can be prevented, a method of controlling the image forming apparatus and a storage medium for the image forming apparatus.